Aqueous solution for controlling bacteria in the water used for fracturing

ABSTRACT

Methods and apparatus of embodiments of the invention relate to a system for treating a subterranean formation including mixing equipment to form a fluid comprising sodium hypochlorite and sodium diacetate; and pumps and a tubular to introduce the fluid into the subterranean formation, wherein a surface of the subterranean formation contains at least 15 percent less microorganisms than if no sodium hypochlorite were in the fluid. Methods and apparatus of embodiments of the invention relate to a method of producing a petroleum product from a wellbore including using a well treatment system comprising mixing equipment, pumps, and a tubular, forming a fluid comprising sodium hypochlorite and sodium diacetate; and introducing the fluid to the well treatment system to achieve a reduced population of microorganisms in the system. Methods and apparatus of embodiments of the invention relate to a system, comprising: a subterranean formation, a well treatment apparatus comprising mixing equipment, pumps, and a tubular, and a fluid comprising sodium hypochlorite and sodium diacetate to achieve a reduced population of microorganisms in the system. Methods and apparatus of embodiments of the invention relate to a method for treating a subterranean formation, comprising forming a fluid comprising sodium hypochlorite, a buffer, and a polymer; introducing the fluid to a surface of a subterranean formation; and decreasing a population of microorganisms, wherein the surface of the subterranean formation contains at least 15 percent less microorganisms than if no sodium hypochlorite were in the fluid, and wherein the fluid exhibits a pH of about 4.0 to about 7.5. Methods and apparatus of embodiments of the invention relate to a method for treating a subterranean formation, comprising forming a fluid comprising sodium hypochlorite and sodium diacetate; and introducing the fluid to a subterranean formation, wherein forming the fluid does not include introducing an acid, and wherein forming the fluid does not include forming a precipitate.

PRIORITY

This application claims priority to U.S. Provisional Application No.61/217,899, filed Jun. 5, 2009, which is incorporated by referenceherein in its entirety.

BACKGROUND

Hydraulic fracturing uses fluid additives such as slickwater additives.The demand for this type of well services has increased over the pastdecade, especially because of its successful application for shale gas.Horizontal wells are often standard, requiring as much as 4.2 milliongallons of water per well in as many as 6 to 9 fracture stages. Becauseof environmental concerns and fresh water availability, the flowback andproduced water are collected and used for subsequent fracturetreatments. Produced water is a perfect environment for sulfate reducingbacteria (SRB) and acid forming bacteria (AFB) due to its anaerobicnature (<2 ppm oxygen content) and high nutrient content (organics, freeiron, etc.). Reuse of water introduces enough oxygen through regularpumping operations to allow aerobic bacteria to grow—mostly slimeforming bacteria (SFB). The oxygen content is high enough for aerobicbacteria to grow but too low to kill anaerobic bacteria. The oxygencontent will cause the anaerobic bacteria to stay in a biostatic statewhich does not kill them but prevents them from multiplying.

As soon as the bacteria find an environment that is conducive to theirgrowth, they will become active again and start multiplying. Theanaerobic environment in the formation is ideal for growth of bacterialike SRBs and AFBs. The aerobic environment of the wellbore is conducivefor SFBs. The growth of SRBs will not only lead to health and safetyconcerns due to increased sour gas or hydrogen sulfide (H₂S) productionbut also to a slow souring of the reservoir. This also increasesoperation expenses because of corrosion (H₂S pitting, stress cracking,etc.) in surface and subsurface tubulars. Other challenges in productioncan be related to AFBs (pitting) and SFBs (emulsion like materials mayform).

Various different methods can be applied to prevent bacteria growth andreduce operational expenses related to corrosion prevention, remediationof corrosion effects, and remediation of emulsion-like produced fluids.Common biocides are quaternary amines, glutaraldehyde,tetra-kis-hydroxylmethylphosphonium sulfate, and tetrahydro3,5-dimethyl-1,3,5-thiadiazinane-2-thione. The issues with traditionalnon-oxidizing biocides like those described above are that they eachhave compatibility issues with common additives in stimulationfracturing treatments (e.g. quat amines are not compatible withquaternary and zircontate crosslinked fluids fluids or anionic frictionreducing polymers) and that they are very toxic. Despite the treatmentof water with these biocides, post-fracture treatment reservoir souringhas been reported. The re-growth of SRB under reservoir conditions maylead to reservoir souring. An effective, low cost biocide that iscompatible with other fluid additives and that is easily transportableis needed.

FIGURES

FIG. 1 is a bar graph of free active chlorine as a function ofhydrochlorous acid concentration for three time periods.

FIG. 2 is a bar graph of bacterial population as a function of time forthree types of bacteria when a fluid comprises a friction reducer.

FIG. 3 is a bar graph of bacterial population as a function of time forthree types of bacteria when a fluid comprises a biocide.

FIG. 4 is a bar graph of bacterial population as a function of time forthree types of bacteria when a fluid comprises a hypochlorous acid.

FIG. 5 is a photograph comparing produced water before and afteraddition of hypochlorous acid.

FIG. 6 is a chart illustrating the percent drag reduction as a functionof rate that compares a fluid comprising a viscosity modifying agentwith and without hypochlorous acid.

FIG. 7 is a chart illustrating viscosity as a function of time for thefluid identified by Table 2 and varied concentrations of hypochlorousacid.

FIG. 8 is a chart illustrating viscosity as a function of time for thefluid identified by Table 3 and varied concentrations of hypochlorousacid.

FIG. 9 is a chart illustrating viscosity as a function of time for thefluid identified by Table 4 and varied concentrations of hypochlorousacid.

FIG. 10 is a chart illustrating viscosity as a function of time for thefluid identified by Table 5 and varied concentrations of hypochlorousacid.

FIG. 11 is a schematic view of mechanical equipment configured toperform an embodiment of the invention.

FIG. 12 is a chart illustrating bacterial population as a function oftypes of bacteria in a field trial comparing the microbe content offresh water, produced water, mix water, mix water and hypochlorous acid,and flowback water and acid after 21 days.

FIG. 13 illustrates viscosity as a function of time for a guar fluidthat contains no sodium hypochlorite and two different concentrations ofsodium hypochlorite.

FIG. 14 shows titration curves for addition of sodium diacetate bufferto various solutions of concentrated industrial sodium hypochlorite intap water.

FIG. 15 shows titrations of some produced water samples treated withsodium hypochlorite (0.21 gpt) and one sample of tap water that waspre-acidified using citric acid prior to treatment with concentratedindustrial sodium hypochlorite.

FIG. 16 shows drag reduction in a 0.5″ pipe using 0.25 gpt frictionreducer, versus water.

FIG. 17 provides friction reduction curves at 0, 15, and 30 minutes.

SUMMARY

Methods and apparatus of embodiments of the invention relate to a systemfor treating a subterranean formation including mixing equipment to forma fluid comprising sodium hypochlorite and sodium diacetate; and pumpsand a tubular to introduce the fluid into the subterranean formation,wherein a surface of the subterranean formation contains at least 15percent less microorganisms than if no sodium hypochlorite were in thefluid. Methods and apparatus of embodiments of the invention relate to amethod of producing a petroleum product from a wellbore including usinga well treatment system comprising mixing equipment, pumps, and atubular, forming a fluid comprising sodium hypochlorite and sodiumdiacetate; and introducing the fluid to the well treatment system toachieve a reduced population of microorganisms in the system. Methodsand apparatus of embodiments of the invention relate to a system,comprising: a subterranean formation, a well treatment apparatuscomprising mixing equipment, pumps, and a tubular, and a fluidcomprising sodium hypochlorite and sodium diacetate to achieve a reducedpopulation of microorganisms in the system. Methods and apparatus ofembodiments of the invention relate to a method for treating asubterranean formation, comprising forming a fluid comprising sodiumhypochlorite, a buffer, and a polymer; introducing the fluid to asurface of a subterranean formation; and decreasing a population ofmicroorganisms, wherein the surface of the subterranean formationcontains at least 15 percent less microorganisms than if no sodiumhypochlorite were in the fluid, and wherein the fluid exhibits a pH ofabout 4.0 to about 7.5. Methods and apparatus of embodiments of theinvention relate to a method for treating a subterranean formation,comprising forming a fluid comprising sodium hypochlorite and sodiumdiacetate; and introducing the fluid to a subterranean formation,wherein forming the fluid does not include introducing an acid, andwherein forming the fluid does not include forming a precipitate.

DETAILED DESCRIPTION

At the outset, it should be noted that in the development of any suchactual embodiment, numerous implementation-specific decisions must bemade to achieve the developer's specific goals, such as compliance withsystem related and business related constraints, which will vary fromone implementation to another. Moreover, it will be appreciated thatsuch a development effort might be complex and time consuming but wouldnevertheless be a routine undertaking for those of ordinary skill in theart having the benefit of this disclosure. The description and examplesare presented solely for the purpose of illustrating the preferredembodiments of the invention and should not be construed as a limitationto the scope and applicability of the invention. While the compositionsof the present invention are described herein as comprising certainmaterials, it should be understood that the composition could optionallycomprise two or more chemically different materials. In addition, thecomposition can also comprise some components other than the onesalready cited.

In the summary of the invention and this description, each numericalvalue should be read once as modified by the term “about” (unlessalready expressly so modified), and then read again as not so modifiedunless otherwise indicated in context. Also, in the summary of theinvention and this detailed description, it should be understood that aconcentration range listed or described as being useful, suitable, orthe like, is intended that any and every concentration within the range,including the end points, is to be considered as having been stated. Forexample, “a range of from 1 to 10” is to be read as indicating each andevery possible number along the continuum between about 1 and about 10.Thus, even if specific data points within the range, or even no datapoints within the range, are explicitly identified or refer to only afew specific, it is to be understood that inventors appreciate andunderstand that any and all data points within the range are to beconsidered to have been specified, and that inventors have disclosed andenabled the entire range and all points within the range.

Embodiments of the invention relate to the use of sodium hypochlorite asan effective biocide in combination with sodium diacetate for use inoperations related to recovering hydrocarbons from subterraneanformations, such as fracturing operations, especially those fracturingoperations that use fluid additives for viscosity modification. That is,embodiments of this invention relate to the use of sodium hypochloriteand sodium diacetate for killing and managing microbes in water used forfracturing including slickwater fracturing. In some embodiments,hypochlorous acid can be delivered in a dilute and stable form, such asby using EXCELYTE™ composition, which is commercially available fromBenchmark of Houston, Tex. Calcium hypochlorite may be selected for someembodiments. It also will form hypochlorous acid upon exposure to water.

Hypochlorous Acid

Generally, when chlorine is added to water, hypochlorous acid is formedaccording to the equation:

Cl₂+H₂

HOCl+HCl

Hypochlorous acid has outstanding bactericidal power. This is generallyattributed to its ability to diffuse through cell walls and therebyreach the vital parts of the bacterial cell. A widely accepted theorycredits the death of the cell to a reaction between hypochlorous acidand enzyme. The hypochlorite ion has little if any bactericidal effectsince its negative charge impedes penetration of the cell wall.

The bactericidal power of a solution of chlorine, a hypochlorite, or achloramine is directly proportional to the hypochlorous acidconcentration of the solution. The percent available chlorine asun-dissociated hypochlorous acid is therefore the true measure of thebactericidal effectiveness of a solution containing one of the chemicalsof the available chlorine family.

The available chlorine family is comprised of the group of chemicalswhich, when dissolved in water, yield solutions of hypochlorous acid.These compounds may be further subdivided into those which contain freeavailable chlorine and those which contain combined available chlorine.

Oxidizing power of a hypochlorite and/or hypochlorous acid solution isattributable to the amount of active oxidant, measured as Free AvailableChlorine (FAC), irrespective of pH. Organic chloramines are also asource of FAC, where the low rate of hydrolysis of dissolved organicchloramines to give hypochlorite and/or hypochlorous acid contributeslittle to the rate of oxidation while maintaining the total oxidizingpower, which relates to the amount of organic chloramines present. Thus,organic chloramines and other reagents that contribute to FAC supplymore hypochlorite and/or hypochlorous acid as these oxidizers aredepleted.

Hypochlorous acid is 25 to 100 times more effective than bleach as adisinfectant without being corrosive. The key active ingredient,hypochlorous acid, is a naturally occurring molecule synthesized from anelectrolyzed solution of salt and water. When exposed to atmosphericconditions, it quickly degrades into saltwater, therefore not leavingecological damage at field locations.

Hypochlorous acid does not fully dissociate and has a neutral pH.(around 7.5). In aqueous solutions, hypochlorous acid partiallydissociates into a salt (the hypochlorite ion), therefore its use in oilfield service application does not leave an undesirable ecologicalfootprint. In contrast, the most commonly used oxidizers do notsterilize and completely kill bacteria. Hypochlorous acid, on the otherhand, reacts quickly with any organic-based or readily oxidizablematerials (Fe, H₂S) present in the water. Further, hypochlorous acid isnoncorrosive compared to other biocides.

In some embodiments, hypochlorous acid will have a concentration ofabout 1 to 8,500 ppm in a fluid. The pH of hypochlorous acid influencesthe free available chlorine concentration. The relationship between pHand the degree of dissociation acid is illustrated by Table 1.Hydrolysis increases rapidly as the pH rises above neutrality.

TABLE 1 Dissociation of Hypochlorous Acid as a Function of pH at 25° C.pH % HOCl Undissociated 5.0 99.6 6.0 96.5 7.0 73.0 7.4 50.0 8.0 21.0 9.02.7 10.0 0.3

Hypochlorous acid may be commercially manufactured using severalmethods. In some embodiments, hypochlorous acid may be made by exposingwater containing sodium chloride to an electrolytic cell. It can also bemade in a more concentrated form in the field by using a buffer, such assodium diacetate, to lower the pH of a sodium hypochlorite solution inwater. Finally, in some embodiments, hypochlorous acid may be generatedby dissolving chlorine gas in water.

Hypochlorous acid can also be formed by introducing sodium hypochloriteinto a solution that has a pH that can be synthesized from anelectrolyzed solution of salt and water, or generated by lowering the pHof a hypochlorite solution to a pH below 7.5, often tailored to have apH of 4 to 7. For example, a continuous process that includes continuousaddition of sodium hypochlorite and pH modifying agent such as a weakacid such as on the fly mixing in oil field service applications may beselected. PH modifying agents such as weak acid, a buffer and/or astrong acid may be used to tailor the pH. In some embodiments, thepreferred pH modifying agent may comprise water-soluble organic acidswith twelve or fewer carbon atoms. The weak acid is an acid thatexhibits a pKa of less than 6. Weak acids include potassium dihydrogenphosphate, phthalic acid, phthalates such as potassium hydrogenphthalate and related acid salts, chelates, citric acid, sulfamic acid,ascorbic acid, octanoic acid, nonanoic acid, propionic acid, erythorbicacid, succinic acid, glutaric acid, adipic acid, polyacrylic acid,maleic acid, cyanuric acid, orthophosphoric acid, acetic acid, andsodium, potassium, and calcium salts of these acids. A weak acid, abuffer, or a combination thereof may be used to tailor the pH. In someembodiments, the preferred weak acid may comprise water-soluble organicacids with twelve or fewer carbon atoms. The preferred weak acidexhibits a pKa of less than 6.

In some embodiments, a pH modifying agent may include a strong acid thatdoes not contain a halogen, such as sulfuric, nitric, or phosphoric acidmay be used in very dilute concentration, such as nanomolarconcentration. Other buffers, buffer solutions, or buffer systems may beselected.

The pH modifying agent may be selected to activate upon the passage oftime or temperature, such that the hypochlorous acid is present insolution after the solution containing sodium hypochlorite and pHmodifying agent is pumped into a wellbore. Generally, however thehypochlorous acid is manufactured, the pH modifying agent may beselected to modify the pH of the fluid over a tailored time ortemperature. Agents most likely to be effective include polylactic acid,polyglycolic acid, or similar hydrolytic polyesters. Delay may beenhanced by isolating the agent in an oil phase and the sodiumhypochlorite in the water phase in some embodiments, the acid may beencapsulated. Upon temperature and downhole mixing, delayed formation ofhypochlorous acid may be achieved. Fumeric acid encapsulated in wax mayalso be selected.

However the hypochlorous acid is formed, to maintain the hypochlorousacid concentration within a fluid, the fluid may be tailored to exhibita pH of 4.0 to 7.5 using a buffer or weak acid. In some embodiments, thepreferred weak acid may comprise water-soluble organic acids with twelveor fewer carbon atoms. A weak acid is an acid that exhibits a pKa ofless than 6. Weak acids include potassium dihydrogen phosphate, thallicacid, phthalates, chelates, citric acid, sulfamic acid, ascorbic acid,octanoic acid, nonanoic acid, propionic acid, erythorbic acid, succinicacid, glutaric acid, adipic acid, polyacrylic acid, maleic acid,cyanuric acid, orthophosphoric acid, acetic acid, and sodium, potassium,and calcium salts of these acids. In some embodiments, a strong acidthat does not contain a halogen, such as sulfuric, nitric, or phosphoricacid may be used in very dilute concentration, such as nanomolarconcentration. Other buffers, buffer solutions, or buffer systems may beselected.

Additional chemicals may be added to a hypochlorous acid composition tostabilize the hypochlorous acid concentration and/or to reduce thereactivity of the bacteria's residual enzymes. Dichloroisocyanuric acid,cyanuric acid, sulfamic acid, potassium iodate,ethylenediaminetetraacetic acid, or a combination thereof may beselected for some embodiments.

The method can also include contacting the aqueous medium with an enzymeactivity minimizer including a metal. In an embodiment, the metal caninclude a heavy metal compound in the aqueous medium including oilfieldproduced water. In an embodiment, the heavy metal can include zirconiumcompound. Zirconium containing chemicals may be used to reduce thereactivity of residual bacteria enzymes. Examples of zirconiumcontaining chemicals that act as enzyme activity minimizer includezirconium nitrate, zirconyl chloride, zirconium phosphate, zirconiumpotassium chloride, zirconium potassium fluoride, zirconium potassiumsulfate, zirconium pyrophosphate, zirconium sulfate, zirconiumtetrachloride, zirconium tetrafluoride, zirconium tetrabromide,zirconium tetraiodide, zirconyl carbonate, zirconyl hydroxynitrate,zirconyl sulfate, zirconium complexed with amino acids, zirconiumcomplexed with phosphonic acids, hydrates thereof and combinationsthereof. Organo-zirconium compound examples include zirconium acetate,zirconyl acetate, zirconium acetylacetonate, zirconium glycolate,zirconium lactate, zirconium naphthenate, sodium zirconium lactate,triethanolamine zirconium, zirconium propionate, hydrates thereof andcombinations thereof. Zirconium dichloride oxide may be selected forsome embodiments.

Other Fluid Additives

The carrier fluid, such as water, brines, or produced water, may containother additives to tailor properties of the fluid. Rheological propertymodifiers such as friction reducers, viscosifiers, emulsions,stabilizers, solid particles such as proppant or fibers, or gases suchas nitrogen may be included in the fluid. The fluid may includeviscosity modifying agents such as guar gum, hydroxyproplyguar,hydroxyelthylcellulose, xanthan, or carboxymethylhydroxypropylguar,diutan, chitosan, or other polymers or additives used to modifyviscosity for use in the oil field services industry. Water based fluidsmay include crosslinkers such as borate or organometallic crosslinkers.In some embodiments, the fluid may contain viscosity modifying agentsthat comprise viscoelastic surfactant. Viscoelastic surfactants includecationic, anionic, nonionic, mixed, zwitterionic and amphotericsurfactants, especially betaine zwitterionic viscoelastic surfactantfluid systems or amidoamine oxide viscoelastic surfactant fluid systems.

Applications

The fluid may be used as a fracturing fluid, drilling fluid, completionsfluid, coiled tubing fluid, sand control fluids, cementing operationsfluid, fracturing pit fluid, or onshore or offshore water injectorfluid, or any other fluid that is introduced into a subterraneanformation primarily for the recovery of hydrocarbons. The fluid isintroduced to the subterranean formation by drilling equipment,fracturing equipment, coiled tubing equipment, cementing equipment, oronshore or offshore water injectors. During, before, or after the fluidis added to a subterranean formation, the formation may benefit fromfracturing, drilling, controlling sand, cementing, or injecting a well.

An oil field services application of a hypochlorous acid fluid mayinclude delivery of the fluid to the following mechanical equipment.Hypocholorous acid fluid may be delivered to the low pressure side ofthe operation, that is, into any low pressure hose, connection,manifold, or equipment; before or during treatment. Examples of thelocation for addition include into pond, pit, or other water containmentsource; into inlet hose/manifold of water tanks (upstream of watertanks); frac tanks—all together or separate; into water tanks (fractanks) themselves; into hose/manifold of outlet side of water tanks;into batch mixing unit; into hose/manifold in between batch mixing unitand blender; into blender itself; into exit side of blender (upstream offracturing pumps); hose/manifold; directly into low pressure side ofpump manifold (missile). Hypochlorous acid fluid may be delivered to thehigh pressure side of an operation including into any high pressureiron, anywhere. Pumps that may be used, either solo or combined, includepositive displacement pumps, centrifugal pumps, and additive pumps. Thehypochlorous acid fluid may be added to the water stream in any way.(i.e. pour from a bucket, pump it into the water, etc.).

FIG. 11 is a schematic view of mechanical equipment configured toperform an embodiment of the invention. Working tanks 1101 contain wateror liquid that is introduced to water line 1102. Water line 1102 mayinclude an entry port 1103 for acetic acid or sodium diacetate or otherpH controlling agent. The entry port 1103 includes a connection to thepH control agent line 1104 which is connected to additive skids 1105,which may be any type of pump or other delivery device. The line 1104may include acetic acid, sodium diacetate, or other pH controlling agentor any other chemical. The skids 1105 are controlled, in part, byfeedback from a control device 1106 as illustrated by lines 1107. Thewater line 1102 is also in communication with a pH meter or other onlineor offline sampling entity 1108 which may be used to determine the pH orother property of the water as it enters water line 1102. The entity1108 sends a signal via a line 1109 or using a transmitter that does notrequire lines to a pH meter 1110 or other property measurement devicewhich sends a signal to the control device 1106 via lines 1111 or usinga transmitter that does not require lines. The entities 1108 and 1112may be any type of probe such as an electrode. The pH meter 1110 alsocollects information from pH meter or other online or offline samplingentity 1112 via line 1114 or using a transmitter that does not requirelines which is connected to a blender 1113. The pH meter 1110 sends asignal via line 1115 or using a transmitter that does not require linesto the control device 1106. In any event, as the water or liquid flowsthrough line 1102, it continues on into the blender 1113 whereadditional chemicals are introduced via line 1116, which delivers sodiumhypochlorite and/or other biocide and/or any other relevant chemical.The delivery of sodium hypochlorite is, in part, controlled by the skids1105, which receive a signal from the controller 1106 via lines 1117 orusing a transmitter that does not require lines. The fluid flows fromthe blender 1113 on to the manifold 1118 via lines 1119 or using atransmitter that does not require lines, then on to the wellbore throughthe pumps or other lines or other equipment of the manifold 1118 and onto tubulars or other wellbore equipment.

In some embodiments, the pH control or component concentration controlof the system may be performed using an electronic control system asdescribed above. In some embodiments, manual control may be used,including measuring the pH and/or composition of the water in the tanks1101 or the line 1102 or the line 1119. In some embodiments, no pHmetering may be performed at all and the concentration of components maybe established based on volume of material. In some embodiments, ahybrid manual/electronic control system may be used with sampling andaddition partially manually controlled, partially electronicallycontrolled. In some embodiments, addition of one component may be usingthe skids 1105 described above or using equipment configured foraddition at other points in the blender, line 1119, or in the manifold.In some embodiments, the controller 1106 and/or pH meter 1110 and/orskids may be the same piece of equipment. In some embodiments, thecontroller 1106 and/or pH meter 1110 may be omitted altogether,especially if the volume of material is fixed. In some embodiments theblender 1113 may be a blender, a tubular, a line, a static mixer, or anyother equipment that may provide static or agitated mixing or blending.In some embodiments, the order of mechanical equipment including mixing,blending, introducing components, measuring pH may be altered. Further,the control system may be configured in alternative ways to accommodatechanges in the mechanical equipment.

In some alternative embodiments, delivering the components to form thehypochlorous acid fluid to the mechanical equipment in the field must beselected based on the source of the acid. Commercially availablehypochlorous acid, such as EXCELYTE™, is delivered premixed into anysize storage containers. It may be added to the system with any way intoany of the above points of addition. Sodium hypochlorite may be combinedwith a weak acid on the fly or by batch mixing. In on the flyapplications, the material may be added by separate add lines—one forsodium hypochlorite, one for acid/buffer (any order); by a combinedsystem—concentrated mixture of sodium hypochlorite and acid/buffer; orby a slurry system—combined mixture of water, sodium hypochlorite andacid/buffer. In batch mixing applications, the components may be mixedtogether before or during the fracturing job and stored in any type ofcontainer. It may be added to the system with any way into any of theabove points of addition. In some embodiments, hypochlorous acid maykill or retard the reproduction of microorganisms. In some embodiments,hypochlorous acid in the fluid will result in a fluid with at least 25percent less microorganisms or at least 25 percent less bacteria than ifno hypochlorous acid were present.

EXAMPLES

The following examples are presented to illustrate the preparation andproperties of fluid systems, and should not be construed to limit thescope of the invention, unless otherwise expressly indicated in theappended claims. All percentages, concentrations, ratios, parts, etc.are by weight unless otherwise noted or apparent from the context oftheir use.

Several analytical tools were selected to confirm the effectiveness ofhypochlorous acid, its compatibility with other fluid additives, and itsstability over time with optional stabilizing additives.

Water Quality—Water Analysis (Produced Water)

The type of water used in these Examples, unless described otherwise,was produced water from the Piceance Basin, which is considered to beamong the dirtiest, recycled, produced water with poor quality. Thesample water was provided to us by a supplier and yielded a pH of 8.0and a TDS of 142,000 ppm. Titrimetric methods were used to determine theanions present while Inductively Coupled Plasma spectrometry was usedfor the detection of cations in the sample water.

Free Available Chlorine (FAC) Demand Test

The chlorine exists in the water as hypochlorous acid (free availablechlorine, FAC). Chlorine is effective against all microorganisms and anyreadily oxidizable organic matter. If there is a lot of organic matterin the fracturing water, the chlorine will be consumed (or spent) andwill be unavailable for killing the bacteria. Therefore, it is necessaryto have a residual of FAC in the water to be effective as a biocide. TheFAC demand test determines the dosage of hypochlorous acid necessary totreat the water and kill the bacteria in the frac water. The FAC demandtest was used to determine the dosage of hypochlorous acid necessary totreat and kill the bacteria downhole. The FAC of the sample water wasdetermined at several time points up to 45 minutes using variousconcentrations of hypochlorous acid. 5% (v/v) of hypochlorous acidsolution containing 500 to 1000 ppm active ingredient was found to bethe lowest effective concentration that showed a positive FAC residualnecessary to sanitize and kill the microorganisms present in theproduced water. FIG. 1 shows data collected on a Piceance Basin watersample. As can be seen, 5% (v/v) or 50 gpt hypochlorous acid solutionwas the lowest effective concentration that showed a positive result ofFAC residual which was necessary to sanitize and kill the bacteria.Consequently, 50 gpt hypochlorous acid solution was used as theconcentration for all subsequent testing regarding hypochlorous acid.

Bottle tests were used to evaluate the biocidal efficacy of hypochlorousacid against the three types of bacteria mentioned above as well ascompare its performance with friction reducer and commonly used biocide,glutaraldehyde. The bacterial population was measured at time points upto seven days.

Effect of Friction Reducer on Bacterial Population

FIG. 2 shows the effect of a viscosity modifying agent, that is, afriction reducer has on biological activity. As can be seen thatfriction reducer had little or no effect on the bacterial population.0.25 gal/Kgal polyacrylamide emulsion was added to the Piceance Basinwater sample for a period of seven days to see its effect on thebiological activity. The above figure shows that the friction reducerhad little or no effect on the bacterial population.

Effect of Gluteraldehyde on Bacterial Population

FIG. 3 shows that glutaraldehyde is not very effective in killingbacteria in Piceance River water containing 0.25 gpt of frictionreducer. Note a 2 log reduction in the population of SRB after 24 hours(a 3 log reduction is desirable). However, after 7 days there wasre-growth of bacteria. 0.25 gal/Kgal glutaraldehyde was added to theproduced water sample along with 0.25 gal/Kgal friction reducer toevaluate the effect of glutaraldehyde on bacterial activity for sevendays. The above figure shows that glutaraldehyde in the presence offriction reducer was not effective in killing the bacteria in the watersample; however, there was a 2 log reduction in the SRB population after24 hours. After seven days, regrowth of bacteria was apparent,suggesting the possibility of sour wells after fracturing treatment.

Effect of Hypochlorous Acid on Bacterial Population

FIG. 4 shows the hypochlorous acid is very effective in killing allbacteria in the Piceance River water. After 7 days, the bacterial countswere blow detectable limits and no regrowth was apparent. 5 (v/v)hypochlorous acid solution (500-1000 ppm active) was added to theproduced water sample containing 0.25 gal/Kgal friction reducer toevaluate the effect of hypochlorous acid activity on bacterial activityfor seven days. Within five minutes, the bacterial population wassignificantly reduced from 10⁶ cells/mL to 10¹ cells/mL. After 24 hours,the SRB population was not detectable and regrowth was not apparentafter seven days.

Compatibility with Slickwater Additives and Piceance River Water

Visual tests were performed to illustrate that there were noincompatibilities between viscosity modifying additives and hypochlorousacid solution (500-1000 ppm active). Also, bottle tests were performed.Bottle tests (deionized water and produced water) were performed withdeionized water and produced water, separately. 5% (v/v) hypochlorousacid solution was added to a series of individual bottles withslickwater additives, including clay stabilizer, scale inhibitor,friction reducer and a microemulsion. The compatibility of hypochlorousacid solution and the slickwater additives were observed at time 0 and 5minutes. No incompatibilities were observed between the slickwateradditives and hypochlorous acid solution in deionized water. Beforeadding hypochlorous acid solution to the produced water, there was astrong rotten egg odor in the water sample indicating the presence ofSRB. After the five minute treatment of hypochlorous acid solution, acolor change was observed and the rotten egg odor was eliminated.Additionally, the pH remained stable for all fluids tested. FIG. 5 showsaddition of hypochlorous acid to the produced water eliminated rottenodor and color changed to a lighter shade. The pH remained stable afterthe hypochlorous acid treatment. Apparently, the hypochlorous acid isvery effective in improving the quality of produced water by oxidizingthe contaminants.

Effects of Hypochlorous Acid on Friction Reducer

A friction loop consisting of a ½″ and a ¾″ pipe was used for dragreduction measurements. Synthetic water was prepared based on the wateranalysis of the Piceance Basin produced water sample. Fifteen liters ofthe source water, along with the slickwater additives and hypochlorousacid solution, were stirred using an overhead stirrer at 1000 rpm fortwo minutes before being added to the friction loop for evaluation.Before analysis, the differential pressure gauges were purged and thepump was primed prior to recording the data for the test. The test fluidwas then pumped for about 10 seconds at incremental intervals of about 6Kg/min and the percent drag reduction was calculated. The figure showsthe friction loop results of the slickwater additives and hypochlorousacid measuring the percent drag reduction as a function of flow rate(Kg/min). Varying the viscosity modifying additives with and withouthypochlorous acid shows no incompatibilities as illustrated by FIG. 6.FIG. 6 shows the friction loop results of slickwater additives andhypochlorous acid. Data are plotted as the percent drag reduction as afunction of flow rate (Kg/min). Hypochlorous acid had no effect on theslickwater additives. This shows that the viscosity difference due tothe presence of the hypochlorous acid in about 2 percent or less.

Hypochlorous Acid in Combination with Common Fracturing Fluids

The compatibility of hypochlorous acid was evaluated with commonfracturing fluids currently used in field operations. The hypochlorousacid solution was used at concentrations of 0 gal/Kgal, 10 gal/Kgal, and50 gal/Kgal. The fluid compositions are listed in Tables 2-5. Fluidswere tested at 150 deg F. for a period of one hour. The mixing procedurefor the fracturing fluids is as follows: 500 mL of deionized water wasplaced into a Waring blending cup; subsequently, the hypochlorous acidsolution was added and allowed to mix for 20 seconds. The gelling agentwas then added and allowed to mix for 10 minutes, after which the lineargel viscosity was checked and compared to the hydration chart (seebelow). The remaining additives were then added to the solution and thevortex was allowed to close (after the addition of the crosslinker).Rheology profiles of the four fluids may be found in FIGS. 7 to 10 whichillustrate the experimental results generated using the fluids of tables2-5. The fluids did not result in a significant loss in viscosity whenthe hypochlorous acid solution concentration was increased from 0gal/Kgal to 50 gal/Kgal. Additionally, the fluid is still viable andcapable of transporting proppant.

Common Fracturing fluids that may be utilized with hypochlorous acid arelisted in the following tables.

TABLE 2 Fluid formulation 1 Additive Concentration Tetramethyl 2 gptammonium chloride (TMAC) Slurried guar 6.25 gpt Borate crosslinker 3 gptHypochlorous acid 0, 10, 50 gal/Kgal solution (500-1000 ppm active)

TABLE 3 Fluid Formulation 2 Additive Concentration Tetramethyl 2 gptammonium chloride (TMAC) Slurried guar 6.25 gpt Boric acid 5.5 pptSodium Hydroxide 10 ppt d-Sorbitol 2 gpt Hypochlorous acid 0, 10, 50gal/Kgal solution (500-1000 ppm active)

TABLE 4 Fluid Formulation 3 Additive Concentration Tetramethyl 2 gptammonium chloride (TMAC) Slurried guar 6.25 gpt Sodium Borate 1.3 gpt30% Sodium 0.5 gpt Hydroxide Hypochlorous 0, 10, 50 gal/Kgal acidsolution (500-1000 ppm active)

TABLE 5 Fluid Formulation 4 Additive Concentration Polyvinyl acetate/6.7 gpt polyvinyl alcohol copolymer Erucic amidopropyl 40 gpt dimethylbetaine Hypochlorous acid 0, 10, 50 gal/Kgal solution (500-1000 ppmactive)

TABLE 6 Linear Gel Viscosities at Increased Biocide ConcentrationsBiocide Concentration Temperature (gpt) (F.) Viscosity (511, sec⁻¹) 071.2 19 10 74.1 18.5 50 68.7 18

Stabilization of Hypochlorous Acid

Using 100 mL of 3% (v/v) bleach, 29 mL of 5% (v/v) acetic acid was addedto obtain a pH of 6.5 from an initial pH value of 8.48. The FAC residualwas greater than 1000 ppm. Additionally, in a separate experiment, 22 mLof 1M sodium citrate was added to the bleach solution to obtain a pH of6.5. The FAC value was then found to be 24 ppm. More details of thisportion of the experimental data are presented below in paragraph 0061.

Bottle tests were used to evaluate the stabilization of hypochlorousacid with the following chemicals: dichloroisocyanuric acid (DCCA) andcyanuric acid (CA). Cyanuric acid is known to stabilize the rate ofdecomposition of hypochlorous acid in ultraviolet conditions. Over aperiod of four days, a set of bottles with the following components wereleft open: 1) hypochlorous acid solution (500-1000 ppm active), 2)hypochlorous acid solution +30 ppm CA, 3) hypochlorous acid solution +50ppm CA, 4) hypochlorous acid solution +30 ppm DCCA, and 5) hypochlorousacid solution +50 ppm DCCA. At the time of preparation, the initial pHand FAC were taken and recorded (see table below). The test points werethen taken again after 1 day and four days. For all solutions prepared,the pH was stable (within 5% of the starting hypochlorous acid solution)after the addition of DCCA and CA. Additionally, the FAC residual valuefor all solutions decreased by 5%, with the DCCA-containing solutionsobtaining a consistently higher FAC residual than hypochlorous acidsolution alone.

Time = 0 Time = 1 day Time = 4 days FAC FAC FAC pH (ppm) pH (ppm) pH(ppm) Hypochlorous acid 6.80 726 6.86 692 7.18 628 solution Hypochlorousacid 6.51 660 6.76 646 7.13 587 solution + 30 ppm CA Hypochlorous acid6.47 670 6.77 637 7.16 580 solution + 50 ppm CA Hypochlorous acid 6.62739 6.75 705 7.13 639 solution + 30 ppm DCCA Hypochlorous acid 6.79 7396.99 705 7.20 639 solution + 50 ppm DCCAHypochlorous Acid Solution Made from Sodium Hypochorite.

A tank is filled with 400 gallons of city water. To this is added 20gallons of 12% sodium hypochlorite solution. This result in a 0.6%solution of sodium hypochorite. To this is added an excess of citricacid until the pH of the resulting solution reaches pH equal to 6.5.This stock solution is then added on the fly to the fracturingtreatment. The concentration of the stock solution added to thefracturing fluid was 0.2 to 0.6 gallons per thousand gallons. Using 100mL of 1% (v/v) sodium hypochlorite (10000 ppm), 12.8 mL of 5% (v/v)acetic acid was added to obtain a pH value of 7.0 from an initial pH of9.7. The active concentration (FAC residual) of the resultant solutionwas then found to be 8500 ppm. After one hour, the active concentrationremained the same. In 24 hours, the active concentration decreased by3.5% to 8210 ppm. Similarly, 55.2 mL of 0.1M succinic acid solution wasadded to 100 mL 1% (v/v) sodium hypochlorite to obtain a pH value of7.0. The active concentration was found to be 6040 ppm after titration.

A fracturing treatment using hypochlorous acid lasted two days. Fourstages, at 2 hours per stage, were pumped using a total of 1.86 milliongallons of water. 1.6M pounds of proppant were used. In total, 19 kgallons of hypochlorous acid solution (500-1000 ppm active) was pumped.The concentration of hypochlorous acid solution that was required (10gpt) also required bulk storage and high rate additive pumps. A 12,000gallon fluid module (modified frac tank) was placed next to the waterfrac tanks. An additive skid with 2 large Waukesha pumps, capable of 45gpm, added hypochlorous acid at a rate of 42 gpm. Hypochlorous acidsolution was pumped from the bulk module tank and into the 250 bbl batchmixing tank.

In another field test, 26 gpt hypochlorous acid solution was added with1 gpt slickwater fluid and mixed for less 1 min at 80 bbls/min to a forma fluid. To be precise, the pH of the fluid was 6. Thus, the 26 gpthypochlorous acid was 2.5 percent active hypochlorous acid and 0.075percent hypochlorite ion. FIG. 12 is a chart illustrating bacterialpopulation as a function of types of bacteria in a field trial comparingthe microbe content of fresh water, produced water, mix water, mix waterand hypochlorous acid, and flowback water and acid after 21 days. Thatis, FIG. 12 illustrates the microbe population demise over time. Alldata points were taken on location. Initial flowback data was taken 21days after job completion. The final flowback data point was taken 51days after job completion.

Sodium Diacetate in Combination with Sodium Hypochlorite

Several tests were performed to illustrate how sodium diacetate works asa buffer for maintaining the pH and thus the integrity of the sodiumhypochlorite. Titrations were performed using Eppendorf-stylemicropipettors to dispense sodium diacetate into 100 ml fluid samplescontained in glass jars. The mixture was continuously stirred at mediumshear by a 15 mm Teflon stir bar actuated by an Ika stir plate. The pHwas measured using a Fisher Scientific XL-15 pH meter that wascalibrated freshly prior to the beginning of each titration. FAC wasmeasured using the Hach spectrophotometer, which colorimetricallyevaluates [OCl]. A friction loop consisting of a ½″ and a ¾″ pipe wasused for drag reduction (DR) measurements. The pressure difference(denoted as ΔP) across the pipes, as well as the mass flow and thetemperature were recorded or each fluid analyzed. Initially, thefriction loop was calibrated with local tap water prior to the fluidtesting and all tests were run at room temperature. Fluid was preparedby adding 0.25 gpt friction reducer to treated water and stirring for 2minutes at 100 rpm using an overhead mixer. After the prepared fluid wasadded to the friction loop hopper, the differential pressure gauges werepurged and the pump was primed prior to recording the data for the test.The test fluid was pumped for about 10 seconds at incremental intervalsof about 6 Kg/min and the percent drag reduction (% DR) is calculatedusing the following equation (Eq 3):

$\begin{matrix}{{\% \mspace{14mu} {DR}} = {\frac{{\Delta \; {Pwater}} - {\Delta \; {Pfluid}}}{\Delta \; {Pwater}} \cdot 100.}} & (3)\end{matrix}$

Each fluid had its friction pressure measured at time 0, at time=15minutes, and at time=30 minutes to gauge the effect of the formationcleaning fluid and buffer on the acrylamide friction reducer. A controlexperiment without buffer and concentrated industrial sodiumhypochlorite was run in a similar manner to evaluate the decrease infriction pressure inherent in running the fluid through the looprepeatedly.

The first round of titrations were performed using variousconcentrations of concentrated industrial sodium hypochlorite in SugarLand tap water to ensure the performance of sodium diacetate buffer wasas expected in the presence of sodium hypochlorite. Severalconcentrations of sodium hypochlorite and other readily availablereagents were tested. The results are summarized in FIG. 14. FIG. 14shows titration curves for addition of sodium diacetate buffer tovarious solutions of concentrated industrial sodium hypochlorite in tapwater.

In the simplest of these experiments, tap water with an initial pH of7.82 has its pH changed to 5.43 by the addition of 0.5 gpt buffer.Addition of a further 0.5 gpt or even another 10 gpt preserve apH.ofjust under 5.4. There is a stir-rate dependence in the experiment—at lowshear, the pH reported by the probe is not quickly representative of thebulk solution because the probe is immersed in ˜2.5″ of a 100 mlsolution. With increased stirring, this feature went away. The same“high shear” stir rate was used in all the other experiments. Severalwater samples containing a level of concentrated industrial sodiumhypochlorite appropriate to water cleaning were tested in the samemanner, and all converge on pH of between 5.4 and 7 with minimaladdition of buffer. Note that all the titrations trend out to final pHvalues between 5 and 7, even when as much as 12.5 gpt buffer is added.

FIG. 15 shows titrations of some produced water samples treated withconcentrated industrial sodium hypochlorite (0.21 gpt) and one sample oftap water that was pre-acidified using citric acid prior to treatmentwith concentrated industrial sodium hypochlorite. FIG. 15 illustratestitration of various acidic and produced waters with sodium diacetatebuffer. Even acidic produced waters can have pH correction up into amore benign range using sodium diacetate buffer.

The objective of friction pressure measurements was to verify whichcombinations of friction reducer, formation cleaning agent (concentratedindustrial sodium hypochlorite), and buffer have little or no effect onthe friction reducing capacity. In order to establish this, a controlexperiment was performed to quantify the effect of recirculation in theloop on the acrylamide polymer in the friction reducer. FIG. 16 showsdrag reduction in a 0.5″ pipe using 0.25 gpt friction reducer, versuswater.

At high rate, the friction reducer reduces friction by about 65%. Testduration is about 3 minutes. The test was repeated after the fluid wassimply left to sit in the loop under static conditions, giving the lower(30 min) trace, which shows ˜61% friction reduction.

This experiment was then performed using a fresh sample with 0.25 gptfriction reducer, 0.21 gpt concentrated industrial sodium hypochlorite,and 0.5 gpt sodium diacetate. The friction reduction curves at 0, 15,and 30 minutes are shown in FIG. 17. It is notable that 30 minutes andtwo cycles of testing cause roughly the same depletion of frictionreducing power when sodium diacetate and sodium hypochlorite are presentas was observed for the friction reducer itself. One interpretation isthat chemical activity of the sodium diacetate and sodium hypochloriteon the friction reducer is negligible as compared to the shear imposedby the test. When extrapolating from a lab scale to a field situation,the shear rates may be considerably higher but the chemistry should bethe same.

From this data, it may be concluded that sodium diacetate buffer cancorrect the pH of a hypochlorite solution in produced water from a highpH to below 5.5. Sodium diacetate buffer can correct the pH of a morestrongly acidic hypochlorous acid solution in produced water from a pHnear 3.0 to a pH of almost 5.0. Sodium diacetate buffer does not have anadverse effect on the stability of concentrated sodium diacetatesolutions at concentrations relevant to slickwater fracturing. In fact,sodium diacetate buffer adjusts pH of alkaline fluids into a range wherethe active water cleaning chemical in concentrated industrial sodiumhypochlorite is more stable than it would be if the fluid were nearer toneutral pH. Sodium diacetate buffer and concentrated sodium hypochloritetogether do not have a measurable effect on the friction reducingability of friction reducer as measured in a friction loop test.

The particular embodiments disclosed above are illustrative only, as theinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails herein shown, other than as described in the claims below. It istherefore evident that the particular embodiments disclosed above may bealtered or modified and all such variations are considered within thescope and spirit of the invention. Accordingly, the protection soughtherein is as set forth in the claims below.

1. A system for treating a subterranean formation, comprising: mixingequipment to form a fluid comprising sodium hypochlorite and sodiumdiacetate; and pumps and a tubular to introduce the fluid into thesubterranean formation, wherein a surface of the subterranean formationcontains less microorganisms than if no sodium hypochlorite were in thefluid.
 2. The system of claim 1, wherein the fluid is introduced to thesubterranean formation by drilling equipment, fracturing equipment,coiled tubing equipment, cementing equipment, or onshore or offshorewater injectors.
 3. The system of claim 1, further comprising equipmentto control the pH to about 4.0 to about 7.5.
 4. The system of claim 1,wherein the fluid further comprises a viscosity modifying agent.
 5. Themethod of claim 1, wherein the fluid further comprises an enzymeactivity minimizer.
 6. A method of producing a petroleum product from awellbore, comprising: using a well treatment system comprising mixingequipment, pumps, and a tubular; forming a fluid comprising sodiumhypochlorite and sodium diacetate; and introducing the fluid to the welltreatment system to achieve a reduced population of microorganisms inthe system.
 7. The method of claim 6, wherein the fluid has aconcentration of 1 to 8,500 ppm hypochlorous acid.
 8. The method ofclaim 6, wherein the well treatment system further comprises drillingequipment, fracturing equipment, coiled tubing equipment, cementingequipment, or onshore or offshore water injectors.
 9. The method ofclaim 6, wherein the introducing the fluid to the system furthercomprises fracturing, drilling, controlling sand, cementing, orinjecting a wellbore.
 10. The method of claim 6, further comprisingadjusting the pH to about 4.0 to about 7.5.
 11. The method of claim 6,wherein the fluid further comprises a viscosity modifying agent.
 12. Amethod for treating a subterranean formation, comprising: forming afluid comprising sodium hypochlorite, a buffer, and a polymer;introducing the fluid to a surface of a subterranean formation; anddecreasing a population of microorganisms, wherein the surface of thesubterranean formation contains less microorganisms than if no sodiumhypochlorite were in the fluid, and wherein the fluid exhibits a pH ofabout 4.0 to about 7.5.
 13. The method of claim 12, wherein the fluid isintroduced to the subterranean formation by drilling equipment,fracturing equipment, coiled tubing equipment, cementing equipment, oronshore or offshore water injectors.
 14. The method of claim 12, furthercomprising controlling the pH to about 4.0 to about 7.5.
 15. The systemof claim 12, wherein the fluid further comprises an enzyme activityminimizer.
 16. A method for treating a subterranean formation,comprising: forming a fluid comprising sodium hypochlorite and sodiumdiacetate; and introducing the fluid to a subterranean formation,wherein forming the fluid does not include introducing an acid, andwherein forming the fluid does not include forming a precipitate.
 17. Asystem, comprising: a subterranean formation, a well treatment apparatuscomprising mixing equipment, pumps, and a tubular, and a fluidcomprising sodium hypochlorite and sodium diacetate to achieve a reducedpopulation of microorganisms in the system.